The invention relates to a method for regulating a high temperature gas phase process, in particular CVD, LPCVD (Low Pressure Chemical Vapour Deposition) or CVI, on the basis of measurement curves, determined by infrared spectroscopy, which have at least one spectral region (peak) characteristic for the regulation of the process and differing from the background of the measurement curve.
Usually, chemical compounds have highly specific infrared spectra which are more finely structured than spectra in the visible or ultraviolet regions. Consequently, infrared spectra frequently serve to assist the qualitative analysis in the identification of chemical compounds, whereby a measured spectrum, which contains the chemical compounds of interest, is compared with a so-called reference spectrum and, by quotient formation/division of the spectra, further absorption spectra are obtained which enable identification of the chemical compounds under examination.
As the spectral regions, which are characteristic for the chemical compounds, i.e. peaks, vary substantially in dependence on temperature, it is necessary that, for each temperature at which a process is carried out, a reference curve is determined. The same holds true when infrared spectra have to be measured and evaluated in different installations.
From U.S. Pat. No. 5,175,017 or JP 07-90593A or JP 03-193863A, process regulation by means of EDV evaluation of spectrometer values is known. An overview of the process regulation possibilities, which also include the spectroscopy, can be obtained from the United States Journal of Vacuum Science Technology B13(4) July/August 1995, 1917-1923. U.S. Pat. No. 4,148,931 proposes a process regulation by IR spectroscopy in exhaust gas.
The present invention is based on the problem of developing a process of the first-mentioned type so that, with the assistance of infrared spectroscopy, high temperature gas phase processes, in particular CVD, LPCVD and CVI processes, can be optimally and quickly regulated even at temperatures which vary during the processes, so that the gas phase process is optimized or the accumulation of, in particular, dangerous chemical compounds in exhaust gas flow is reduced.
According to the invention, the problem is solved, essentially, in that a straight line (synthetic background) is calculated directly from the measurement curve on the basis of the initial and end values of the characteristic spectral range (peaks), and in that the regulation of the process occurs by integration of the spectral range above the straight line or by determination of the maximum height of the spectral range over the straight line or on the basis of another characteristic value of the spectral range with respect to the straight line.
It is thereby in particular provided that the measurement curve is smoothed out, i.e. the background noise is strongly minimized, before the calculation of the straight line. The smoothing can be effected in accordance with the formula established by Kinitz
Ai={fraction (1/16)}(A(ixe2x88x922)+4A(ixe2x88x921)+6A(i)+4A(i+1)+A(i+2)),
where A1 represents the value to be smoothed (i.e. in the present case the peak) and A(ixe2x88x92x) or A(i+x) the value before or after the value Ai.
By the teachings of the invention, the infrared spectrum can be continuously established and evaluated independently of a spectral background which varies with temperature, so that a regulation of the process is possible while avoiding a reference spectrum at each temperature to be measured. Consequently, immediately after the calculation of the straight line, which is equivalent to a synthetic background, relevant parameters of the process can be regulated on the basis of characteristic values obtained between the straight line and the peak. These include the pressure in a reaction vessel, the gas speed of the process gas, the concentration thereof and/or the temperature in the reaction chamber.
On the basis of the teachings of the invention, it is no longer necessary, after reinstallation, on transferring to other process installations and in particular other or varying temperatures to determine so-called background data banks for the different temperatures before the actual measurements in order to divide the actually measured measurement curves by reference curves thus obtained. Consequently, without any adaptation, the gas composition can be determined even while a process is taking place. This is in particular the case when the positions of the characteristic spectral ranges, i.e. the leading peaks, are known, so that consequently, independent of parameters and also independent of the high temperature gas phase processes to be carried out, an immediate calculation and therewith regulation of the process can occur. In this way, PyC or CVD, LPCVD or CVI installations, e.g. for PyC or SiC coatings, can be optimized or controlled in a problem-free manner.
The method according to the invention can be used, in particular, for regulating a waste, such as a refuse, incineration process, a spectral range of the measurement curve characteristic for an environmentally dangerous gas such as dioxin being the basis for regulating values.
In a CVD process (Chemical Vapour Deposition) for surface coating of e.g. carbon or graphite material with silicon carbide, characteristic peaks measured in the exhaust gas flow, such as HCl and/or CH4 and/or CH3SiCl3 and/or HSiCl2 and/or SiCl4 can preferably be the basis for the regulating values.
For determining the measurement curve, IR radiation emitted from an element such as graphite plate in a reaction container, can be measured, the IR radiation, in particular, emitted through an exhaust gas flow from the reaction vessel being the basis. In this way, by adjustment of the process parameters, it can be ensured that e.g. the amount of polychlorosilanes in the exhaust gas is reduced to such an extent that expensive subsequent treatments are avoided. Also, the separating apparatus can be regulated so that, consequently, the CVD process is optimized by means of the method according to the invention. The same holds true with respect to the optimization of CVI (Chemical Vapour Infiltration) processes and pyrographite coatings (PyC).
The method according to the invention is not, however, restricted to infrared measurements in which the radiation is emitted from a body. Moreover, all known infrared spectroscopy methods employing emission, transmission or reflection spectra can be used.